Japanese unexamined patent application publication No. JP-A-2007-101636 describes a related art image forming apparatus. In the related image forming apparatus such as an electrophotographic printer, there are known so-called tandem type color printer in which photosensitive drums corresponding respectively to colors such as yellow, magenta, cyan and black are disposed in parallel.
In this tandem type color printer or image forming apparatus, developing rollers are provided so as to correspond respectively to the photosensitive drums. By the photosensitive drums and the developing rollers being driven to rotate in such a state that the developing drums are in engagement with the photosensitive drums, toner is supplied to latent images formed on the photosensitive drums from the developing rollers, whereby toner images are held on the photosensitive drums. The toner images corresponding to the colors of the photosensitive drums are formed on the photosensitive drums, and the toner images in the respective colors are transferred onto a sheet in a superimposed fashion color by color, whereby the formation of a full color image on the sheet is achieved. In addition, a black toner image is formed only on the photosensitive drum corresponding to black, and by the black toner image being transferred onto a sheet, the formation of a monochrome image on the sheet is achieved.
Since no toner image is formed on the other yellow, magenta and cyan photosensitive drums than the black photosensitive drum when a monochrome image is formed, the corresponding developing rollers are desirably disengaged or spaced apart from the yellow, magenta and cyan photosensitive drums so as to prevent the wear of the developing rollers.
Then, as such a tandem type image forming apparatus, there is proposed an image forming apparatus including engagement and disengagement members which can move in a straight line in a direction in which photosensitive drums are arranged, so as to switch the position of corresponding developing rollers to an all color photosensitive drums disengagement state in which the developing rollers are disengaged or spaced apart from all the photosensitive drums, a black photosensitive drum disengagement state in which the developing roller is left in engagement with the black photosensitive drum, while the other developing rollers are disengaged or spaced apart from the yellow, magenta and cyan photosensitive drums, and an all color photosensitive drums engagement state in which the developing rollers are in engagement with all the photosensitive drums.
The engagement and disengagement members extend in the direction in which the photosensitive drums are arranged and each have a flat upper surface. Cam portions having a trapezoidal shape as viewed from the side are formed on the upper surface of the engagement and disengagement members. The cam portions each have a sliding surface which is inclined from the upper surface of the engagement and disengagement member obliquely upwards towards a predetermined direction which follows along the direction in which the photosensitive drums are arranged (referred simply to as a “predetermined direction” in this section and the following section entitled “Summary”) and a flat spacing surface which extends from an end of the sliding surface in the predetermined direction in parallel with the upper surface of the engagement and disengagement member. In addition, projecting portions are formed on a developing cartridge so as to project from an upper end portion thereof in a direction which intersects the predetermined direction at right angles. The projecting portions confront the corresponding engagement and disengagement members from thereabove.
In such a state that the projecting portions are placed on contact planes constituted by the upper surfaces of the engagement and disengagement members, the developing rollers are disposed in an engagement position where the developing rollers are in engagement with the corresponding photosensitive drums. When the engagement and disengagement members are caused to move in an opposite direction to the predetermined direction from the state described above, the projecting portions move from the contact planes to the sliding surfaces of the cam portions, slide over the sliding surfaces and thereafter ride onto the spacing surfaces of the cam portions. By this series of actions, the developing cartridges are raised by the projecting portions, and the developing rollers are disposed in a disengagement portion where the developing rollers are disengaged or spaced apart from the corresponding photosensitive drums. On the other hand, when the engagement and disengagement members are caused to move in the predetermined direction from this state, the projecting portions move from the spacing surfaces to the sliding surfaces, slide over the sliding surfaces and are allowed to return to the contact planes. By this series of actions, the developing cartridges move downwards, whereby the developing rollers are disposed in the engagement position where the developing rollers are brought into contact with the corresponding photosensitive drums.